text: AR4 is expressed in Halobacterium species xz 515. The organism was first identified in a salt lake in Tibet. The gene encoding it was identified by H. Wang and colleagues in 2000. In most bacteriorhodopsin homologs, H release to the extracellular medium takes place before a replacement ion is taken up from the cytosolic side of the membrane. Under the acidic conditions found in the organism's native habitat, the order of these stages in the AR4 photocycle is reversed.